Since the 13th century, firearms have operated on the principle that an explosive mass of powder, generally referred to as gun powder, could be ignited and caused to react and “explode” causing a sudden increase in pressure within a confined and defined space. This constant volume pressure increase was caused to happen behind a projectile, which was then forced in the one direction it could move, along with the exploding gas, which was down a barrel and out the end of a firearm muzzle. Early firearms were loaded down the muzzle, by first inserting a charge of gunpowder, and then on top of that powder adding a projectile, which was typically a 
As firearm technology progressed, primarily in the United States during the 1850's and 1860's, it became possible to load a charge of powder into a casing, or shell, and seat the projectile in a friction fit at the open end of the casing. This discovery lead to the development of a whole new era in firearm development. Christopher Spencer received patent protection on Mar. 6, 1860 (U.S. Pat. No. 27,393) for what became known as the Spencer Repeating Rifle, Tyler Henry received a patent for the Henry Rifle on Oct. 16, 1860 (U.S. Pat. No. 30,446), and Horace Smith and Daniel Wesson eventually formed Smith & Wesson to manufacture some of the first revolvers using these new cartridges, and thereby continued firearm development which led to the issuance of numerous patents for innovation during this time period. Of course, Colt's Patent Manufacturing Company received a large number of patents over the years, perhaps most notably for its Colt's Single Action Army Revolver which utilized these new cartridges in what is now a famous revolving cylinder repeater.
All of these developments in firearm and cartridge technology paved the path from muzzleloaders to the modern cartridge, which, even today, is typically comprised of a metal casing (originally copper and now often brass), with a primer lodged in one end and the bullet (projectile) lodged in the other. Contained within the casing is the gunpowder. The primer does not come out of the casing during the firing of the cartridge. The cartridge is loaded into a modern firearm in a number of different ways depending upon the particular action of the firearm used. The common link between the many modern actions, however, is that they are loaded at their breech, instead of down the muzzle as was traditionally done.
In these more modern firearms, when the firing pin of the firearm strikes the cartridge's primer, the primer ignites the powder within the shell, causing an extremely rapid pressure increase, which causes the projectile to dislodge from the shell's open end, driving the projectile down the barrel of the firearm and out the end of the muzzle toward its target. The explosion is an extremely fast exothermic chemical reaction that occurs in a constant volume as the contents of the gunpowder react. This constant volume expansion causes both a pressure increase and a concomitant temperature increase within the system. It is the large and extremely rapid pressure increase during the chemical reaction of the powder that generates the force necessary to drive the projectile at a high speed down the barrel. 
Many modern loads have been developed to generate bullet energies over 3,000 ft-lbs at the muzzle and bullet velocities over 3000 ft/sec at the muzzle. For example, a typical 150 grain 0.30-06 bullet will have a muzzle velocity of about 2900 ft/sec and hold nearly 2900 ft-lbs. of energy at the muzzle. This level of energy requires powders and loads that generate great temperatures and pressures within the barrel. As the high temperature gases follow the bullet down the bore of the barrel, the temperature of the barrel raises significantly. This is especially profound when rapid-fire rifles are involved because the barrel does not have time to cool between shots.
One problem resulting from this combination of high pressure and temperature is an increase in the wear of the barrel, and as a result, reduced barrel life. Because pressure is greatest at the breach end (gas volume increases linearly while the physical volume increases exponentially and pressure is equal to gas volume divided by physical volume), the deterioration occurs more rapidly at the breach end of the barrel. This problem is exacerbated with higher pressure cartridges. Thus, heat dissipation is most beneficial to barrel life in the breach end of the barrel.
Also a problem is the high recoil of the high-pressure, heavy bullet systems common today. Recoil is essentially defined as what the shooter experiences as he holds the firearm, often to his shoulder, and always at least in his hand or hands, as the firearm discharges. For every action, there is an equal and opposite reaction. If a 200 grain bullet leaves a muzzle with over 3000 ft-lbs of energy, that momentum is also applied through the firearm to the shooter holding the firearm. These great recoils are not only sometimes uncomfortable or even damaging to the shooter, but greatly affect accuracy, target reacquisition, and sight realignment between shots.
Still another problem with these modern loads, particularly in tactical situations, is with respect to muzzle flash and report. Muzzle flash and report are essentially visual and audible indications, respectively, of the location of a shooter. By reducing either or both, the exact location of a shooter is less likely to be determined by those around him. Muzzle flash occurs when still-burning powder escapes the muzzle behind the bullet as the bullet exits the muzzle. As it exits and continues to burn (react) the fire or flash indicated can give away shooter location, especially at night or low light conditions. The problems with sound are, of course, obvious. One that merits detailing is that the greater the muzzle report, the more  likely the shooter, or shooters near to the shooter, will flinch in anticipation of the loud, harmful sound, causing a decrease in the marksmanship of the shooter.
Some developments have occurred to attempt to remedy some of the above-described problems. Baffle muzzle breaks, for example, work on the principle of redirecting gases that would otherwise exit the muzzle in the direction of the projectile. In such cases, their performance is proportional to the percentage of gas they deflect. Many such muzzle breaks redirect expanding gases in a direction perpendicular to the longitudinal axis of the bore of the firearm, or in an angled, rearward direction at an acute angle with respect to the longitudinal axis of the bore of the firearm. In such cases, noise and debris is directed toward the shooter's face. Problems with this scenario are also obvious, not the least of which is increased potential for damage to the shooter's, or nearby person's, eardrums, and pronounced shooter's flinch resulting in a further degradation of marksmanship.